Recently, in the field of optical communication or optical measurement, optical modulators have been widely used in which an optical waveguide is formed on a substrate having an electro-optic effect and a modulation electrode including a signal electrode and ground electrodes to control an optical wave guided in the optical waveguide is formed on the substrate.
Such optical modulators require various characteristics according to various usages, one of them is frequency dependency of optical response characteristics in a high frequency band over 10 GHz, that is, it is required that frequency characteristics are excellent. However, as shown in FIG. 1, generally, an optical modulator is provided with an input end portion 31 and an output end portion 32 of a signal electrode on the side of a substrate 1, to introduce and propagate a modulation signal to the signal electrode 3, the input end portion 31 is connected to a high frequency modulation signal introducing path 6, and the output end portion 32 is connected to a terminator 7. Accordingly, a part of an introduced high frequency modulation signal is emitted in the substrate to cause a resonant effect, and thus a significant decrease (ripple) in frequency characteristics occurs at a specific frequency in the high frequency band.
In Patent Document 1, to suppress such a problem, jumper conductors 91 and 92 are provided on the opposite side between which the input end portion 31 or the output end portion 32 of the signal electrode and a waveguide 2 are interposed, and the jumper conductors are connected to a metal case 10 housing the optical modulator.
In FIG. 1, Reference Numerals 4 and 5 denote ground electrodes, and Reference Numerals 81 to 86 denote bonded gold ribbons. In FIG. 1, the optical waveguide and the electrode are displayed to overlap with each other in order to facilitate understanding of the positional relation of the optical waveguide and the modulation electrode.
Patent Document 1: Japanese Patent Publication No. 3731622
One of the important characteristics required for optical modulators is suppression of temperature drift. Generally, the signal electrode and the ground electrodes are not linear-symmetric with respect to a center line of an optical waveguide in a propagating direction of light, except for a part of optical modulators. For this reason, the shape of the signal electrode or the ground electrodes is varied by the difference in thermal expansivity of the substrate or the electrodes caused by variation in temperature in the course of the operation of the optical modulator, and thus internal stress affected on the optical waveguide by the electrodes is also varied. As a result, the refractive index of the optical waveguide is varied, and thus a phase of a light wave propagated on the optical waveguide is affected. Therefore, an operation point shift occurs. The operation point shift caused by such variation in temperature is called temperature drift.
In Patent Document 2, to suppress such a problem, as shown in FIG. 2, paying attention to electrodes on optical waveguides 21 and 22 formed on a substrate 1, it is proposed to control the shape of a signal electrode 3 or ground electrodes 4 and 5 so that the electrodes on the optical waveguides are symmetric with respect to the center (chain line A) of the optical waveguide. Specifically, the shape of the ground electrode 4 is divided into a part 41 corresponding to the signal electrode 3 and a part 43 corresponding to the ground electrode 5, and the parts 41 and 43 are electrically connected by a conductive thin film 42.
In FIG. 2, Reference Numeral 11 denotes a buffer layer.
Patent Document 2: Japanese Patent Application Laid-Open No. 2001-4967.